Stewart R. Wallace — 1919–2009 - Society of Economic Geologists

14 SEG NEWSLETTER No 78 JULY 2009
... from 13
Discovery and Geology of the PGE Deposits of the Bushveld Complex, South Africa (Continued)
plateau and coastal lowlands). In contrast,
the western and northern limbs
are covered by thick soils on the
Highveld. Knowledge of these areas is
largely based on mining activity and
extensive drilling programs, most
notably in the western limb, where mining
has revealed details of some of the
economic reefs over many tens of kilometers.
Despite excellent overviews of
the Rustenburg Layered Suite (Wager
and Brown, 1968; Von Gruenewaldt et
al., 1985) our understanding is inhibited
by subjective interpretion of field
relationships between widely separated
chambers which are unlikely to be
interconnected.
The eastern and western limbs are
subdivided into sectors on the basis of
regional differences in the stratigraphy,
although this has never been formalized.
Sectors are separated by structurally
complex boundary areas.
Regular changes in the stratigraphy of
the southern sector of the western limb
led Eales et al. (1988) to recognize different
facies (broadly referred to as
proximal and distal) on the basis of
(theoretical) feeder localities. This interpretation
is probably widely applicable
and can assist with our understanding
of similarities between, for example, the
distal components of both the western
and eastern limbs. Sectors are further
subdivided into subchambers, typically
from detailed knowledge of chromitite
layers and PGE reefs. In the northern
sector of the western limb, for example,
the Union (Viljoen et al., 1986a) and
Amandelbult (Viljoen et al., 1986b)
mines comprise discrete subchambers.
Facies changes may in part be ascribed
to synBushveld tectonism (Scoon and
Teigler, 1994), despite a conflict with a
widely held belief that layered intrusions
occur in stable continental settings
(e.g., Irvine, 1982). Structural disruptions
in the Bushveld include
diapirism of the floor rocks (Uken and
Watkeys, 1997), and development of
the Rustenburg Layered Suite on the
flanks of domes was influenced by associated
episodic uplift (Scoon, 2002). This
is ascribed to multiple phases of
magma replenishment—an important
tenet of our understanding of the
Rustenburg Layered Suite, as proposed
and discussed by Eales et al. (1988).
The Rustenburg Layered Suite attains
a maximum thickness of some 9 to 12
km within the western and eastern
limbs, whereas the sequence in the
northern limb is significantly truncated.
On a smaller scale, each sector (and
each subchamber) reveals considerable
variability (South African Committee
for Stratigraphy, 1980). The unconformable
relationship with the floor
rocks, which is most apparent in the
eastern limb (Sharpe, 1981), results in
the lowermost parts of the Suite
sequence being spatially restricted in
their development, with only the uppermost
layers being laterally extensive.
Various parental magma types have
been identified to explain the development
of the Suite (e.g., Eales, 2002),
over and above the effects of upward
fractional crystallization (Wager and
Brown, 1968), although the mechanisms
of magma intrusion are poorly
constrained. Scoon and Teigler (1994)
and Uken and Watkeys (1997) suggested
that the thermal and loading
effects of new influxes are important in
enhancing floor rock irregularities,
which must be accounted
for in the interpretation of lateral
variations between sectors
and subchambers.
The Rustenburg Layered
Suite is subdivided into zones
on the basis of a repetitive
cyclicity and laterally extensive
marker layers (Fig. 3). The Mar -
ginal zone consists of relatively
fine grained norite and feldspathic
pyroxenite with little discernible
layering. The Lower
zone is almost entirely ultramafic
and is dominated by layers
of dunite, harzburgite, and
orthopyroxenite. It is the
Critical zone (Hall, 1932), however,
that reveals the most spectacular
layering and contains
the PGE deposits and chromitite
layers. The modern practice is
to recognize a Lower Critical
zone, dominated by feldspathic
orthopyroxenite, and an Upper
Critical zone, with more complex
layering of lithological
units, typically including feldspathic
orthopyroxenite and
norite-anorthosite. Chromitite
layers, which are a defining
characteristic of both the Upper
and Lower Critical zones, are
subdivided into Lower, Middle,
and Upper groups. Chromite
mining is mostly concentrated
on the somewhat higher grade layers in
the Lower Critical zone and lower parts
of the Upper Critical zone (Schurmann
et al., 1998). The Main zone that overlies
the Critical zone is characterized by
relatively monotonous sequences of
norite and gabbronorite, albeit with
prominent layers of anorthosite. The
Upper zone includes numerous Ti magnetite
layers intercalated with magnetite
gabbro, anorthosite, and ferrodiorite.
PLATINUM DISCOVERIES
Details of the original discoveries of
platinum are summarized here from
the review by Scoon and Mitchell
(2004a), which in turn was based on
articles and editorial comments in the
popular mining press of the time (e.g.,
Merensky, 1925), as well as on a biog -
raphy of Hans Merensky (Fig. 4) by
Lehmann (1955). Exploration started
FIGURE 3. Generalized vertical section of the layered
sequence from the eastern limb of the
Bushveld Complex, depicting zonal subdivisions
and important marker layers.